doc: minor argument formatting in stream.markdown

Fixes: https://github.com/nodejs/node/issues/4350
PR-URL: https://github.com/nodejs/node/pull/6016


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>